Embedded radar and electronic warfare equipment comprise microwave electronic devices such as fixed or variable gain low-noise amplifiers (LNA) and vector phase shifters.
These microwave devices usually have balanced microwave signal inputs and sometimes have to be connected to unbalanced signal ports, for example, as in the case of signals originating from a reception antenna that are conveyed and transmitted by an unbalanced coaxial line.
The unbalanced/balanced input conversion of reception microwave devices is usually implemented by an inductance-based passive coupling transformer called a BALUN, BALUN being a contraction of BALanced/UNbalanced.
Such a BALUN-type transformer at the input of a microwave device is accompanied by a significant increase in the footprint thereof at frequencies of a few GHz. Furthermore, the since the BALUN transformer is generally tuned to a predetermined frequency, it consequently operates in narrow band mode and limits the operating band of the microwave device with which it is associated. Furthermore, the noise factor of the receiver is degraded by the losses of the BALUN.
Other transistorized active-type solutions, to balance the signal input, are used in the microwave devices of the prior art. These solutions are based on GaAs (gallium arsenide) technology which offers the advantage of exhibiting very good intrinsic linearity and noise performance levels but, on the other hand, does have the drawback of a high fabrication cost with a low possibility of integration with its ancillary transistor control and biasing circuits. Consequently, these limitations are slowing down the possibility of miniaturizing such circuits. Furthermore, these drawbacks complicate the architecture of the microwave systems and penalize the cost of the equipment.
The use of SiGe (silicon-germanium) type technologies to implement microwave functions, techniques developed mainly for civil applications such as GPS (global positioning system), UMTS and WLAN, provide the benefit of a reduction in the cost of the integrated circuits including these microwave functions, allowing, among other things, for an integration of the interfaces and of the biasing functions for the transistors of the chip, the elimination of the negative potential power supplies, the reduction in consumption of the circuits, a high fabrication yield and good performance reproducibility.
FIG. 1 shows a schematic diagram of a basic differential conversion circuit of the prior art consisting of a differential pair of transistors T1, T2 in common emitter configuration. The emitters E of the two transistors T1, T2 are linked by two feedback impedances Ze in series. The biasing of the two transistors is provided by a perfect current Ig generator SC connected between the common point of the two feedback impedances Ze and the reference potential M. The perfect current generator SC comprises an impedance Zg between its two terminals to simulate a real current generator.
The voltage biasing of the transistors T1, T2 by the collectors C is performed by a device not represented in FIG. 1.
The collectors C of the two transistors T1, T2 are connected to a low-impedance load (not represented in FIG. 1) through a matching stage CHBP (10) of cascode circuit type with balanced outputs s1, s2.
The differential amplifier is dynamically driven in an unbalanced manner by a voltage v1 generator Ge between the base b1 of the transistor T1 and the reference potential M. The base b2 of the transistor T2 is set to the reference potential M.
The main drawback with this type of differential amplifier of the prior art is the degraded common mode rejection because of the impedance Zg, which results in the imbalance between the collector currents as a function of the operating frequency F.